1. Field of the Invention
This invention relates to a technology for producing carbon nanotubes.
2. Description of the Related Art
Japanese Patent Application Publications No. 2003-171108 (JP-A-2003-171108), No. 2006-27947 (JP-A-2006-27947) and No. 2003-277031 (JP-A-2003-277031) disclose a chemical vapor deposition method (CVD method) as a process for producing carbon nanotubes. In the CVD method, a silicon substrate having an outer surface supporting a catalytic metal such as iron (Fe) is placed in a furnace. After the furnace has been heated to a high temperature of about 800° C., a raw material gas such as a hydrocarbon is fed to the substrate so that carbon atoms from the thermal decomposition of the raw material gas are produced and arranged in a tubular form. A multiplicity of carbon nanotubes are then grown upwards from the substrate surface on which the catalyst is supported.
In general, there is a limitation to the length of carbon nanotubes that can be formed on a substrate by the CVD method (hereinafter such a limitation in length will be referred to as “formable limit length of carbon nanotubes”). Spun fibers of carbon nanotubes are manufactured by twisting carbon nanotubes grown up to their formable limit length. However, the obtained carbon nanotube spun fibers may cause a reduction of strength in portions at which individual carbon nanotubes are joined. Thus, there has been a demand for production of carbon nanotubes that have greater formable limit length to increase the strength of spun fibers of carbon nanotubes so far.